The German orbital X-ray telescope ROSAT began to see significant signal changes in the soft X-ray region of the spectrum in the 1990s, lasting for a few days. These resembled the abundant stream of soft X-rays from Comet Hyakutake found around the same time.

It was suggested that these were caused by the solar wind, fluxes of charged particles emanating from the sun, and how they interacted with neutral ions in the geocorona, the upper part of our atmosphere. The distinctive spectra of these events, known as Solar Wind Charge Exchange Events (SWCX), were verified by more thorough studies in the 2000s, and the process became widely recognized.

However, modeling how solar wind gives rise to the measurements of orbital telescopes proved much more difficult. It requires successfully capturing the arrival of solar wind events, how the charged particles interact with neutral atoms and how that affects the magnetosphere.

Researchers at Tokyo Metropolitan University used numerical techniques to predict the fluctuations in soft X-ray signals picked up by X-ray satellites. They examined data from the Suzaku telescope and compared it to simulations of how solar winds would interact with the upper regions of our atmosphere.

By doing so, they were able to capture how the signal varied with the satellite’s orbital motion, with implications for how to make predictions for future satellite experiments.

A highlight of the team’s work is the wide range of astrophysical events they can bring together and how to map them to real data.

A remarkable achievement in the field, the model showed excellent agreement with experimental data, mimicking the observed signal up to a factor of two. In addition, they were able to reproduce the unusually powerful signal fluctuations when the satellite’s line of sight coincided with the polar cusps. There were some notable outliers, such as when a severe geomagnetic storm was seen; yet the ability of the variations to be successfully reproduced holds great promise for predicting the results of the next generation of space X-ray observations.

Magazine reference:

  1. Daiki Ishi et al. Modeling geocoronal solar wind charge exchanges detected with Suzaku. Publications of the Astronomical Society of Japan, psac095, DOI: 10.1093/pasj/psac095